11
$\begingroup$

With today's state-of-the-art electronic systems in modern aircraft, do aircraft equipped with autopilot systems, still require pilot rudder input to keep a turn coordinated?

For the context, I'm assuming the pilot is hand-flying the aircraft so the autopilot must be at least partially disengaged to let the pilot manually initiate and perform the turn via the yoke/stick, but still make the rudder use unnecessary or optional.

For example, would an F-16 pilot still need to apply the rudder during a turn? If the military case is not generic enough, how about an airliner pilot? Or a GA aircraft like the Cirrus SR22?

$\endgroup$
20
$\begingroup$

Most transport aircraft use yaw damper systems to take care of minor rudder inputs. Most autopilot systems are actually only 2-axis - pitch and roll since the rudder's job is only to keep the tail lined up behind the nose. The yaw damper is a separate "autopilot" system and has limited authority, sufficient to deal with minor yaw disturbances, dampen dutch roll, and counter adverse yaw from the ailerons and is active all the time whether the autopilot is on or not.

With a Y/D system the only time a pilot really needs to make a rudder input in flight is if an engine quits, because the yaw damper's authority is not sufficient to counter asymmetric thrust. Generally in a jet, once through the departure profile, feet are on the floor even when hand flying. They are only on the pedals to steer the nosewheel during the takeoff and be ready in case an engine quits.

On a swept wing aircraft the yaw damper is essential; if the yaw damper is off and yaw disturbances occur, dutch roll motions can start and if the pilot tries to respond with rudder inputs he almost always can't stay in phase and things get exciting. Because of this criticality the Y/D is usually dual channel. One of the tests done on production aircraft is to induce a large yaw movement with both YD channels off to get a dutch roll going, then engage each YD channel and make sure it stops the dutch roll.

$\endgroup$
  • 3
    $\begingroup$ Yaw dampers or any stability augmentation system, like pitch dampers or yaw dampers have nothing to do with coordination of a turn, they only dampen or reduce oscillations in that axis of motion due to disturbances. $\endgroup$ – Charles Bretana May 8 '18 at 17:17
  • 5
    $\begingroup$ No. The Y/D system senses slip or skid and operates the rudder, within a limited range, usually around 1/4 of the total available travel, to eliminate slip or skid. Whether it's damping yaw oscillations from a disturbance or reacting to adverse yaw it's the same. The Y/D system doesn't care WHAT created a slip or skid, it just does whatever it has to do to center the ball/brick. If you roll into a turn and the aileron displacement yaws the nose away from the roll a bit, the Y/D system senses the sideslip and moves the rudder to eliminate the slip. You do not need to touch the rudder. $\endgroup$ – John K May 8 '18 at 20:01
  • $\begingroup$ A bit of an exception: Airbus aircraft are full 3-axis autopilots. The rudder control has a separate turn coordination function that feeds forward roll into the rudder, and on FBW, a sideslip function to automatically stabilize the aircraft during engine failure. A major reason behind the complexity is to dampen the longitudinal flex modes of the fuselage, so the aircraft can be longer and less rigid without causing everybody in the back to get airsick. $\endgroup$ – user71659 May 8 '18 at 23:47
  • $\begingroup$ Actually, in spite of the fact that common usage is to describe it as a Yaw or sideslip compensator, it does not do that. It senses yaw rate and applies rudder to oppose that yaw rate. It is a yaw rate stability augmentation system. If you establish a fixed, stable sideslip angle with the rudder, the yaw damper will not oppose it. It dampens yaw oscillations not yaw. $\endgroup$ – Charles Bretana May 9 '18 at 0:49
  • $\begingroup$ With no rudder input from the pilot, a yaw dampener will act to oppose the yaw oscillations we call dutch roll, but if you just attempt to roll into a bank by applying aileron, the adverse yaw from the ailerons will not be opposed by a yaw dampener. Only the yaw rate (while the sideslip is being established) will be counteracted. Once the sideslip has been established, the yaw dampener inputs go away and the sideslip will remain. $\endgroup$ – Charles Bretana May 9 '18 at 0:53
9
$\begingroup$

In my Cessna Cardinal, the rudder is cross coupled to the yoke through some kind of spring/bungey cord system (I don't have the manual handy), so I can fly with my feet off the pedals most of the time. That's from a 1968 design, keeping turns coordinated with no autopilot needed.

$\endgroup$
  • $\begingroup$ +1 and yes, I'm aware of that, but I assume there is a good reason for anything that is not popular in aviation, so I guess there was a reason that that design didn't really take off too. That's why I directed my question toward autopilot systems. $\endgroup$ – Mahdi May 8 '18 at 13:35
  • 5
    $\begingroup$ @Mahdi By definition, the design had to take off... it is not an aircraft if it never takes off. :) $\endgroup$ – T.J.L. May 8 '18 at 13:52
  • $\begingroup$ Which design? The Cessna Cardinal, or the smooth flying that the interconnected rudder offers? Maybe other non-autopilot equipped planes offer similar. $\endgroup$ – CrossRoads May 8 '18 at 14:43
  • 1
    $\begingroup$ @CrossRoads BTW, I'm not saying it's a bad idea or it won't work! I'm just thinking that there must be a reason that that design is not very popular, possibly some added design complexity drawbacks. $\endgroup$ – Mahdi May 8 '18 at 14:56
  • 1
    $\begingroup$ @Mahdi: Perhaps the reason is that most private pilots learn to fly in aircraft that don't have such systems, so that use of the rudder becomes natural. If you then go to a system that eliminates the rudder, you have to consciously think about it. I would think it would be like me trying to drive a car with automatic transmission: unless I pay close attention, I automatically try to use the clutch, and often wind up stepping hard on the brake instead. $\endgroup$ – jamesqf May 10 '18 at 5:40
4
$\begingroup$

The need for rudder to coordinate a turn is directly dependent on Angle of Attack (AOA). At positive angles of attack, the down aileron is more in the relative wind than the Up aileron (due to blanking by the wing in front of the aileron). An aircraft that is at zero AOA, (like a fighter unloaded, in a zero-G ballistic arc) requires no rudder. To avoid this problem, in modern aircraft, like the F-15, for example, differential stabilator is used to mitigate this issue. The stick is mechanized so that the further aft it is, the more any lateral motion is directed to generating asymmetrical stabilator deflection, rather than aileron deflection. SO, at high AOA (assuming that stick position is an accurate indicator of AOA), when the pilot moves the stick to the side, the ailerons deflect very little or not at all, but the stabilators at the tail deflect asymmetrically.

The F-16 flight control surfaces are completely computer controlled, so pilot inputs are interpreted as commands for aircraft movement. The computer then determines, based on all known factors (AOA, Airspeed, etc.) what to do with all control surfaces (including the leading and trailing edge flaps) to get the airframe to move in the manner commanded by the flight control inputs.

I never flew the F-16, (perhaps someone who did can clarify), but my guess is that there is no need to depress the rudder pedal in the F-16 to coordinate a turn - that the computer automatically determines how much rudder, or differential stabilator, should be deflected to coordinate the requested roll rate.

$\endgroup$
  • $\begingroup$ In this podcast around 0:57 the F-16 pilot says 90% of a mission he will not use the rudder, it flies coordinated without input. $\endgroup$ – fooot May 10 '18 at 20:09
  • $\begingroup$ An interesting corollary to this is that early in the F-16's operational deployment, an aircraft was lost due to something called Inertial Roll-Coupling. Common practice in Air-to-Air training when adversaries were on different frequencies (like in a Red-flag type major exercise) in order to acknowledge you were aware of an attack from an adversary, was to perform a rapid wing-rock, rolling the aircraft rapidly back and forth through a large range of bank angles. Apparently the Engineers did not include this scenario in the FBW algorithms, and an aircraft at Nellis was lost as a result. $\endgroup$ – Charles Bretana May 11 '18 at 17:10
4
$\begingroup$

I once spent a couple hours flying a full-motion 737 simulator with an instructor for that type. My previous flight experience comprised entirely one hour at the controls of a Cessna 180 (cruise only) and hundreds of hours of Microsoft Flight Simulator with just a joystick, so I was pretty excited to show off that I knew, in theory, what a coordinated turn is. The very first flight set us up in a situation necessitating a decisive turn to enter the traffic pattern correctly. Being somewhat overwhelmed (hand-flying a 737 in a real sim is a fairly big leap from on a desktop computer), I of course completely forgot I had rudder pedals until halfway through the turn. The moment I even brushed my foot onto one, the instructor said, and I remember this exactly, "woah there; we only use those if we want to make all the passengers sick!" We continued the session hand-flying approaches in all kinds of weather in all kinds of places. I never once needed to touch the rudder pedals again.

The modern Boeing jets all have the ability to use their yaw-damper systems to coordinate turns (although turn coordinating is physically not the same as yaw damping). The details of the implementation depend on the model. For example, 747s coordinate turns only with the flaps down, while 777s and C-17s always do.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.